1. Field of the Invention:
The present invention relates to encoders used to physically sense the position of optical apparatus and other apparatus. More specifically, the invention relates to systems used to maintain the internal alignment of encoders.
While the present invention is described herein with reference to an illustrative embodiment for a particular application, the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings of the present invention will recognize additional modifications, applications and embodiments within the scope thereof.
2. Description of the Related Art:
The positioning system of a spacecraft scanning mirror is a typical example of the many applications of encoders. Encoders are useful where it is desirable to know, with a high degree of accuracy and certainty, the position of an apparatus mounted for movement about a shaft of pivot. In a typical encoder a transparent disc is mounted on the shaft between a light source and a photodetector. An optical pattern is superimposed on the periphery of the disc between the source and detector. The pattern includes a first set of tracks to provide an indication of the coarse position of the shaft and a second set of tracks to provide an indication of the fine position of the shaft. When the shaft is in motion, the coarse and fine tracks cause the detectors to output bits of a digital word. In a typical system, the least significant bit of the fine track F.sub.LSB would change state once per scan drive step. The most significant bit of the fine track and the least significant bit of the coarse track C.sub.LSB would change state every two scan steps. By design, the least significant bit of the coarse track C.sub.LSB is intended to change state exactly one step out of phase with the most significant bit of the fine track F.sub.MSB. The coarse and fine numbers are then merged into a single binary number which represents the position of the shaft.
The conventional technique for the merger of the coarse and fine numbers involves the comparison of the least significant bit of the coarse track C.sub.LSB to the most significant bit of the fine trace F.sub.MSB. If C.sub.LSB equals F.sub.MSB, then a value N is set equal to the coarse word C. If C.sub.LSB does not equal F.sub.MSB, the N is set equal to C +1. The combined word S is then determined by discarding the least significant bit of N and appending the fine word F.
The coarse and fine numbers may be so merged so long as the 1 to 0 transitions in C.sub.LSB occur within the region where F.sub.MSB is zero. If the 1 to 0 transition in C.sub.LSB drifts out of this range, an incorrect value may occur in the calculated encoder position represented by the combined word S. The accuracy of the system is thererefore dependent on the degree of misalignment of the coarse and fine tracks. If the misalignment is more than one step of the finest resolution, this conventional technique may break down.
Age, wear and other effects cause substantial misalignment between the coarse and fine photodetectors of practical systems. To avoid the resulting degradation in mirror pointing accuracy and possible lock of the mirror control loop, a second encoder is often provided as a spare. For these and other systems, it is desirable to know the extent of misalignment and the rate at which it is increasing. This would allow for a better understanding of decalibrating effects and for a projection of the useful life of the satellite. Thus, there is a need in the art for a system that provides an indication of the degree of internal misalignment of an encoder.